public void Return <T>(IEnumerable <HashSet <T> > items)
 => HashSetConcurrentPool <T> .Return(items);
 public void Return <T>(HashSet <T> item)
 => HashSetConcurrentPool <T> .Return(item);
 public void Return <T>(params HashSet <T>[] items)
 => HashSetConcurrentPool <T> .Return(items);
 public HashSet <T> HashSet <T>()
 => HashSetConcurrentPool <T> .Get();